The present invention relates, in general, to shape memory alloys (SMA""s) and, more particularly, to SMA materials that can be used in sensors to detect strain.
Shape memory alloys (SMA""s) are a unique class of materials that have the ability to form two different crystalline phases, usually referred to as the martensite and austenite phases, in response to temperature and strain. SMA""s are produced by combining at least two component elements into a desired shape which is then annealed. Immediately upon being annealed, the SMA material is in the austenite phase, having a specific shape (referred to hereinafter as the parent shape), and characterized by a low ductility, high Young""s Modulus and high yield stress. Upon cooling, the SMA material changes into the martensite phase characterized by a high ductility, low Young""s Modulus and low yield stress. In the martensite phase, the SMA material is easily deformed and can take on a different shape from its austenite, or parent, shape by the application of an external strain thereto. The SMA material will retain this different shape until it is heated to its austenite phase transformation temperature. When such heating occurs, the SMA material undergoes a phase transformation to its austenite phase and is transformed back to its parent shape. During this phase transformation the SMA material produces a very high kinetic energy output per unit volume. Because of this, SMA""s can generate a relatively large force over a longer displacement as compared to other materials of the same size. Additionally, because of the electrical resistance characteristics of SMA material, joule heating can be used to raise the SMA material to its austenitic phase transformation temperature. Furthermore, the electrical resistance characteristics of SMA material results in a strain-dependent electrical resistance effect at the phase transformation temperature.
The two significant physical properties of SMA material, i.e., high recoverable strain and high actuation energy densities, have led to the development of SMA materials and devices for various applications. Bulk or thick film SMA materials are produced using traditional metal forming processes and are incorporated into many different devices ranging from orthodontia appliances to visored helmets. In these applications the bulk or thick film SMA materials take the form of wires, springs, thread fasteners, ring clamps, etc. Thin film SMA materials are produced by depositing an alloy on a substrate and have gained acceptance in micro fluidics and temperature related applications, particularly as actuators. Typically, applications utilizing bulk or thick film SMA materials exploit the one-way shape-memory property of SMA material. In these applications the bulk or thick film SMA material is strained (deformed) in the low temperature martensite phase and recovers to its parent shape upon being heated to the temperature at which the SMA material is transformed to its high temperature austenite phase. The strain-dependent electrical resistance effect of SMA material, however, has not been utilized for strain measuring devices or sensors because of the thermodynamic inefficiency of bulk or thick film SMA material. The hysteresis characteristics of bulk or thick film SMA material, which determine the phase transformation cycle period, are too slow (on the order of seconds) to be effective as a sensor. The slowness of the hysteresis characteristics of bulk or thick film SMA material is caused by the high thermal mass of this material. In contrast, due to the low thermal mass of thin film SMA material, the hysteresis characteristics of this material are quite fast (on the order of cycles/second) which makes thin film SMA material particularly suitable for certain applications, such as sensors, where the change in electrical resistance at a phase transformation of this material can be correlated to a change in strain being applied to the material. A problem, however, arises with thin film SMA material due to the difficulty in producing reliable thin film SMA materials that can repeatedly and consistently provide accurate strain measurements. Recent advances in manufacturing techniques, however, have resulted in the production of thin film SMA materials that exhibit a consistent quality suitable for use in strain measuring devices. Because of these manufacturing advances, it has become desirable to develop a sensor and method for measuring strain utilizing thin film SMA materials.
The present invention provides a method and sensor for detecting strain using shape memory alloys. The sensor comprises a substrate material, a flexible diaphragm provided on the substrate material and a sensor member deposited on the flexible diaphragm. The sensor member is formed from a thin film SMA material and is capable of undergoing a phase transformation, such as from its martensite phase to its austenite phase, in response to a physical stimulus, such as strain, being applied thereto. During such a phase transformation, the electrical resistance of the thin film SMA material undergoes a substantial change. This change in electrical resistance can be correlated to a change in strain being applied to the thin film material. In this manner, the magnitude of the strain can be determined. The present invention also provides a method for measuring a physical stimulus comprising the steps of providing a sensor comprising a thin film SMA material; measuring a physical property, such as the electrical resistance, of the thin film SMA material immediately before the material undergoes a phase transformation caused by the application of a physical stimulus thereto; applying a physical stimulus to the thin film SMA material causing the material to undergo a phase transformation; measuring a physical property, such as the electrical resistance, of the thin film SMA material immediately after the material undergoes a phase transformation; determining the difference in the value of the physical property, i.e., the electrical resistance, that occurs during the phase transformation; and utilizing the difference in the value of the physical property to determine the magnitude of the physical stimulus being applied to the thin film SMA material.